A little applied physics:
Force = Mass X Acceleration
The second part of the equation is usually the easier of the two to see large amounts of improvement on. That is working on hand speed. Simply punch in a way not to emphasize power, but to emphasize the amount of acceleration you put into it. If you try to "put it all" into a punch, you'll tense up. You need to punch in a fluid, loose manner. How? Repitition, repitition, repitition... Keep punching until your arms feel like they're about to fall off.
An easy thing to do, is to punch using either weights on your wrists, or using big ass 16oz training gloves. Once you take the gloves off, your hands will feel lighter, and they will move at a greater rate of acceleration.
Okay, now the first part of the equation takes a lot of fine tuning. The way that you add mass to your punch is simply by using good body mechanics to train yourself to move you body with each punch. Again, the only way to tune your body movements well enough is repitition.
For instance, when you shoot out a jab, turn your hip towards your target. That will cause your shoulders to rotate into the direction of the punch (thus adding the mass of your upper body into the equation). You can add the mass of your lower body into the equation by pushing off your rear foot and sliding your front foot forward two or three inches. This will greatly increase the amount of force in your jab.
I can probably go on in length about it, but because of time constraints let me suggest a few things:
1) Pick up a good reference book that breaks down the kinesthetics of a punch in an easily understandable manner. I suggest two books that I have right off the bat:
The Tao of Jeet Kune Do
by, Bruce Lee (I know, it may be a little dated or philosophy heavy, but the way Lee breaks down the kinesiology of punching is absolutely great)
by, Mark Hatmaker (A more practical and up-to-date reference and instructional book on MMA striking)
Both books are easeily found for under $20.
2) Get a heavy bag and start working rounds... And more rounds... And more rounds.
3) Find a good boxing gym. These are still fairly easy to find, and even moderate amounts of instruction can make a big difference in comparison to finding out what works for yourself. Aside from that, once you begin learning how to throw a KO punch, you're going to want to learn how to avoid getting hit by one.